In many passive antenna assemblies, it is often desired to be able to adjust a radiation pattern of the antenna assembly after the antenna assembly has been installed on a tower. The need may arise due to a number of factors, including new construction, which may create obstacles, vegetation growth, or other changes in the surrounding environment. It may also be desired to alter the radiation pattern due to performance studies or to alter the shape of the area the antenna covers.
There are various ways that the radiation pattern may be altered. One method is to physically change the location of the antenna assembly. Once the assembly has been installed on a tower, however, this becomes difficult. It is also possible to change the azimuth and elevation of the individual antennas, but such a method is expensive when applied to several antennas. Also, the mechanical device required to adjust the azimuth and elevation may interfere with the mechanical antenna mount.
Another method that has been utilized to adjust the radiation pattern of a number of antennas grouped onto one antenna assembly is to alter the phase angle of the individual antennas. By altering the phase angle of the individual antennas, a main beam (which causes the radiation pattern) is tilted relative to the surface of the earth. The antennas are grouped into a first group, a second group, and a third group. All three groups are disposed along a panel of the antenna assembly. A phase adjuster is disposed between two of the antenna groups, such that an adjustment of the phase adjuster changes the radiation pattern. The phase adjuster comprises a conductor coupled with a transmission line to create a capacitor. The conductor is rotatable and moves along the transmission line, changing the location of the capacitor on the transmission line. The transmission line is coupled to an antenna which has a phase angle. The phase angle is dependant partially on the location of the capacitor. Thus, by changing the location of the capacitor, the phase angle is changed. The phase adjuster may be coupled to a plurality of antennas and acts to adjust the phase angle of all of them.
The phase adjusters currently in use, however, have numerous drawbacks. First, the conductor is often made of brass which is expensive to etch and cut. Therefore, the conductor is usually cut in a rectangular shape. The path of the transmission line, however, is arcuate. The conductor does not cover the entire width at the capacitor, which decreases the effectiveness of the capacitance.
Another problem with current phase adjusters is the coupling of a power divider to the phase adjuster. The antenna assembly receives power from one source. Each of the three groups of antennas, however, has different power requirements. Thus, power dividers must be connected to the assembly. Currently, a power divider may be a series of cables having different impedances. Using a variety of cables makes manufacturing difficult since the cables have to be soldered together. Also, since manual work is required, the chances of an error occurring is increased. Another method of dividing the power is to create a power divider on a PC board and then cable the power divider to the phase adjuster. Although this decreases some costs, it still requires the extensive use of cabling, which is a disadvantage.
A third problem is caused by the use of cable lines having different lengths to connect an antenna to the appropriate output from the phase adjuster. Each antenna has a different default phase angle when the phase adjuster is set to zero. The default phase angle is a function of the cable length coupled with the length of the transmission line. To achieve the differing default phase angles, cables of varying lengths are attached to different antennas. Although this only creates a slight increase in manufacturing costs since cables of varying lengths must be purchased, it greatly increases the likelihood of error during installation. In numerous antenna assemblies, the cable lengths only differ by an inch or less. During assembly, if a cable is not properly marked, it may be difficult for the person doing the assembly to tell the difference between the different sizes of cable.
To move the phase adjuster, an actuator is located on a side of the panel and may include a small knob or rotatable disc for manually changing the phase adjuster. Thus, whenever the radiation pattern needs to be adjusted, a person must climb the tower and up the side of the panel to the phase adjuster. This is a difficult and time consuming process. Also, it is only possible to move the actuator manually, requiring the exertion of physical labor. In addition, it is a dangerous activity since the antennas are located on a tower and it is possible for a person to fall or otherwise become injured in the climbing process.